1. Field of the Invention
The present invention relates to a method for controlling data transmission in accordance with the quality of a communication line, and particularly relates to a method of controlling a transmission interval of a xe2x80x9cproper patternxe2x80x9d which is inserted at the top of a plurality of HDLC frames, by monitoring HDLC responses included in received signals. Here, HDLC is an abbreviation of High Level Data Link Control Procedure. The xe2x80x9cproper patternxe2x80x9d is called a xe2x80x9cUnique Wordxe2x80x9d.
2. Background Art
The HDLC is a communication procedure which is internationally standardized in order to efficiently execute communication and communication control between data terminals such as computers and engineering work stations which constitute a network.
FIG. 1 illustrates a plurality of HDLC frames headed by the unique words (UW), and in the present application, a plurality of HDLC frames are called xe2x80x9can HDLC frame groupxe2x80x9d or xe2x80x9cmulti-framesxe2x80x9d. The unique words are used for detecting the head of each frame group and for determining the timings of transmission and reception.
Each HDLC frame which constitutes an HDLC group (multi-frames) headed by respective unique words is composed of, as shown in FIG. 1, a start flag (F), an address field (A), a control field (C), an information field (I), a frame check sequence field (FCS), and an end flag (F).
The commands of frames or the content of responses are designated in the control field (C). Naturally, the HDLC command is included in the transmitted signals, and the HDLC response is included in the received signals. Examples of the responses related to the present invention include RR (Receive Ready), RNR (Receive Not Ready) or REJ (Reject). The transmitting or receiving data is inserted in an information portion (I) frames, and the data length is variable.
If the RNR response is included in the receiving frame, since it means that the opposite data terminal (an opposite station) is not ready to receive data, the data transmission will not be executed. In contrast, if the RR response is included in the receiving frame, since it means that the opposite data terminal is ready to receive data, the data transmission is executed. If the REJ response is included in the receiving frame, the data transmission is executed again since the opposite terminal request re-sending of data.
However, in conventional multi-frame HDLC data transmission, the transmission interval of unique words has been fixed at a constant value, irrespective of a quality of a communication line. Thus, this conventional example (hereinafter, referred to as the first conventional example) has had problems which will be described in a later section.
In contrast, Japanese Patent Application, First Publication No. Sho 63-20933 discloses a technique for optimizing the length of the HDLC frame according to the quality of the communication line (hereinafter, referred to as the second conventional example). According to this conventional technique, a line quality monitoring portion obtains an error generation rate per unit time based on xe2x80x9cthe amount of error control processingxe2x80x9d, and the frame length is determined so as to minimize the data transmission time in response to the thus obtained error generation rate.
Furthermore, Japanese Patent Application, First Publication No. Sho 62-163438, discloses a technique in a packet exchange system using the HDLC procedure, which evaluates the quality of the communication line periodically by measuring the number of frames corrupted due to bit errors, and transmitting the data after dividing the data into packets having short data lengths, if the quality of communication line is not satisfactory (hereinafter, called the third conventional example).
In the above first conventional example, the transmission interval of the unique words UW has been fixed and is not been related to the quality of the transmission line. Thus, even if the transmission quality is degraded and the REJ response (which is equivalent to the demand for re-sending) arrives from the opposite data terminal, the bit length of the unique word and the series of the following HDLC (a transmission unit) is maintained at the same length as that of the case of a malfunction caused just before the reception of data. Under such a circumstance, it is likely that the REJ responses are repeatedly generated, which results in causing the problem of reducing the transmission efficiency.
In contrast, the second and third conventional examples control the length of the HDLC frame itself in response to an assumption of the line quality based on the error generation number. However, the error generation number is not directly interrelated with the line quality; thus, the problem arises that the frame length is sometimes reduced to become unnecessarily short.
It is therefore an object of the present invention to provide a transmission system which is capable of solving the above problems associated with the conventional techniques, since the present transmission system judges the quality of the communication line by monitoring HDLC responses issued by the opposite data terminal and controls the transmission intervals of the unique words.
According to the first aspect of the present invention, the present invention provides a method for controlling data transmission in accordance with the quality of a communication line comprising transmitting a plurality of frames defined by the High Level Data Link Control Procedures (HDLC) by dividing with unique words, the method comprising the step of: changing the interval of transmitting the unique words in accordance with the HDLC response included in the received signal.
Examples of the HDLC responses include RR (Receive Ready), RNR (Receive Not Ready), and REJ (Reject).
According to the second aspect of the present invention, the present invention provides a transmitting and receiving system for transmitting and receiving a plurality of frames defined by the HDLC by dividing unique words, the system comprising: a HDLC monitoring circuit for detecting said HDLC response included in the received signals; a unique word generating circuit for outputting a selected unique word among a plurality of unique words in response to the HDLC response detected by said HDLC monitoring circuit; a multiplexing circuit for multiplexing the unique word selected by said unique word generating circuit with the transmitting data; and a transmitting circuit for transmitting the output of said multiplexing circuit.